1. Field of the Invention
The present invention relates to a process for the preparation of aqueous formaldehyde solutions, from which most of the heat of condensation and absorption released during their preparation is withdrawn and used for evaporating the methanol/water/air mixture required. Furthermore, some of the heat content of the formaldehyde solution produced can be utilized for the recovery of methanol which is contained in the formaldehyde solution. Both measures can be used either individually or simultaneously.
Formaldehyde is an important basic chemical, for example for the preparation of phenol-formaldehyde resins or urea-formaldehyde resins, and is furthermore used as disinfectant and as tanning agent.
1. Description of the Related Art
The preparation of formaldehyde by oxidative dehydrogenation of methanol has been known for a long time. In this reaction, completely evaporated mixtures of methanol, water and air, the composition of which can vary within wide limits outside the range of explosive mixtures, are reacted over catalysts at elevated temperature. Suitable catalysts are silver catalysts and others. The gaseous mixture which leaves the catalyst stage is then converted in absorbers into aqueous formaldehyde solutions. In this reaction, a considerable portion of the heat content of the gases to be absorbed and the heat of absorption and condensation have to be removed. It is therefore desired to utilize this energy which is obtained at a low temperature level in an economic manner. The methanol/water/air mixture required for the preparation of formaldehyde solutions is usually produced in an evaporator by heating with steam.
Surprisingly, it has now been found that the complete evaporation of the methanol/water/air mixture can be carried out in a suitable evaporator below the boiling point of the methanol/water mixture and that the temperature of the aqueous formaldehyde solution formed in the absorber is sufficient for heating.
It is further desired not only to remove most of the methanol from the formaldehyde solution prepared and to reuse it, but also to remove some of the water, in order to have a more concentrated formaldehyde solution available, for example, to save transport costs. This removal of methanol and water is carried out by distillation, preferably under reduced pressure. Such a distillation under reduced pressure is advantageously carried out as flash evaporation, utilizing the heat contained in the crude formaldehyde solutions (EP 100,809). The more the pressure in such a flash evaporation is reduced, the further the column temperature drops, which corresponds to a steadily increasing and desirable utilization of energy. However, this utilization of energy has its limit in terms of economics, since at lower temperatures the condensation of the vapours is only possible by means of highly enlarged condenser surfaces and by using expensive coolants. This cancels the economic advantage of the utilization of energy and eventually turns it into a disadvantage.
Furthermore, it has now been found that the use of expensive coolants can be omitted even at very low pressures in the flash evaporation column, if the condensation of the vapours is carried out as injection condensation by means of liquid water and the resulting methanol-containing aqueous solution which has a low formaldehyde concentration and therefore has to be disposed of, is fed into the catalytic dehydrogenation of methanol instead of the water, which is required as a starting material.
The possibility of operating at very low column pressures also makes it possible to carry out both measures simultaneously and thus achieve optimum utilization of the heat contained in the formaldehyde solution.
The advantages of such a procedure are as follows:
1. saving of the steam required for the evaporation
of the methanol/water/air mixture;
2. saving of methanol while utilizing most of the
energy, in particular in the lower pressure range
of the flash evaporation so that the process can
be managed without external energy;
3. the possibility of also using methanol/water
mixtures of lower concentrations and
4. the possibility of doing without a too high methanol conversion.
The measures mentioned in 3. and 4. promote the selectivity of the catalytic dehydrogenation and thus the yield and are only possible because an extensive recovery of methanol is possible according to the invention in an economic manner.